A Comparison Between Measured and Computed Assessments of the RF Exposure Compliance Boundary of an In-Situ Radio Base Station Massive MIMO Antenna

Advanced antenna technology in the form of massive multiple input multiple output (mMIMO) antennas has been deployed in fourth generation (4G) long term evolution (LTE) mobile networks and will be a standard feature in fifth generation (5G) networks. A characteristic of mMIMO is beamforming in which a large array of antenna elements is used to concentrate energy in specific narrow directions to improve data throughput. Assessing the electromagnetic field (EMF) compliance boundary around a beamforming capable mMIMO antenna using a traditional approach that assumes maximum power is simultaneously transmitted in every possible direction and the radio base station (RBS) is fully loaded (100% traffic utilization) may produce very conservative results. In this study we report on measurements of the 6-minute time-averaged EMF levels in front of a mMIMO antenna operating in a 4G LTE time division duplex (TDD) network and compare results with conservatively computed EMF levels. The RBS serves users scattered throughout a dense urban environment characterized by low rise commercial/industrial buildings. When scaled to 100% RBS traffic loading and taking TDD operation into account, the highest measured EMF level was 7.3% to 16.1% of the ICNIRP occupational reference level compared to a conservative computed level of 79.3%. This represents a far-field reduction in the occupational compliance boundary of between 2.2 and 3.3 times and is consistent with other statistically based theoretical studies. An additional reduction in the compliance boundary can be expected due to actual RBS traffic loading often being much less than 100%.